1. Field of the Invention
This invention relates to an image processing method and apparatus for emphasizing predetermined frequency components of an image signal. This invention particularly relates to automatic setting of an image processing parameter for obtaining desired results of image processing.
2. Description of the Prior Art
The applicant has proposed various image processing methods and apparatuses, wherein frequency emphasis processing is carried out by using an unsharp mask image signal, and a radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, is thereby obtained. (Such techniques are described in, for example, U.S. Pat. Nos. 4,315,318 and 4,317,179.) The unsharp mask image signal represents an image, which is constituted of the same number of picture elements as that of the picture elements of the original image represented by an original image signal, and has sharpness lower than the sharpness of the original image. The unsharp mask image signal has the frequency response characteristics such that the high frequency components of the original image signal, which are not lower than a predetermined frequency, may have been removed.
With the frequency emphasis processing, an unsharp mask image signal Sus is subtracted from an original image signal Sorg, and the obtained difference value is multiplied by an emphasis coefficient .beta.. The resulting product is then added to the original image signal Sorg. In this manner, predetermined frequency components of the original image signal Sorg can be emphasized. The frequency emphasis processing may be represented by Formula (1) shown below. EQU Sproc=Sorg+.beta..times.(Sorg-Sus) (1)
wherein Sproc represents the signal obtained from the frequency emphasis processing, Sorg represents the original image signal, Sus represents the unsharp mask image signal, and .beta. represents the emphasis coefficient.
With the frequency emphasis processing described above, problems often occur in that an artifact occurs due to the addition of the signals. The problems can be solved by adjusting the frequency response characteristics of the addition signal, which is added to the original image signal Sorg. A technique for adjusting the frequency response characteristics has been proposed in, for example, U.S. Ser. No. 08/723,313.
In the proposed technique for adjusting the frequency response characteristics, firstly, a plurality of unsharp mask image signals, which represent images having different levels of sharpness, i.e. which have different frequency response characteristics, are formed from the original image signal. Thereafter, a calculation is made to find the difference between two signals, which are among the original image signal and the unsharp mask image signals. The calculation is made for each of sets of two signals, which are among the original image signal and the unsharp mask image signals. In this manner, a plurality of band-limited image signals (hereinbelow often referred to as the band pass signals), each of which represents the frequency components of the original image signal falling within a certain limited frequency band, are formed. Further, the band pass signals are restricted to desired levels by using different converting functions, and the plurality of the restricted band pass signals are integrated. In this manner, the aforesaid addition signal is formed. The processing thus carried out may be represented by, for example, Formula (2) shown below. ##EQU1## wherein Sproc represents the processed image signal, in which the high frequency components have been emphasized, Sorg represents the original image signal, Susk (k=1 to N) represents the unsharp mask image signal, f.sub.k (k=1 to N) represents the converting function for converting each band pass signal, and .beta.(Sorg) represents the emphasis coefficient determined in accordance with the original image signal.
With the proposed technique for adjusting the frequency response characteristics, the frequency response characteristics of the addition signal, which is to be added to the original image signal Sorg, can be adjusted by altering the definition of each of the converting functions f.sub.1 to f.sub.N for converting the band pass signals. Therefore, in order for an artifact to be prevented from occurring, the converting functions f.sub.1 to f.sub.N may be defined as functions such that an image signal having frequency response characteristics causing no artifact to occur can be formed. Further, besides the purposes for preventing an artifact from occurring, a processed image signal having desired frequency response characteristics can be obtained in accordance with the definition of each converting function.
However, in cases where each converting function is to be defined actually in accordance with the technique described above, how each converting function is to be defined such that desired results can be obtained cannot be found easily from a formula, such as Formula (2) shown above. Therefore, heretofore, each converting function has been defined by adjusting a parameter (e.g., the inclination of the function), which defines the converting function, little by little while an image obtained from the emphasis processing is being seen. The defining work is difficult for users having little experience to carry out. Even for users experienced in such work, considerable time and labor are required to carry out the defining work.